Many optical instruments, especially those used in ophthalmology, require fixation of a person's head relative to the optical and mechanical structure of the instrument. A commonly used approach is to provide one or more surfaces against which the patient rests his forehead and chin (see FIG. 1 for example). Given the relatively large and unpredictable variability in the shape of heads and chins, this presents a challenge to the designer. The designer must provide adjustable positions for the surfaces that can be set for each patient and then locked to prevent further movement. This is conventionally done by including linkages and sliding rods with lever or screw operated brakes.